This invention relates to the nuclear-reactor art and has particular relationship to the processing of the radioactive gases which are generated as a result of the fission which takes place in a reactor. In the interest of concreteness, but not with any intention of limiting the scope of this invention, a pressurized-water (PWR) reactor is here considered. Among the important fission products of such a reactor are the radioactive isotopes xenon (Xe) 131m (metastable) 133, 133m, 135, and 135m and krypton (Kr) 85, 87, 88 and 85m (metastable). The Xe isotopes have half-lives as follows:
131m -- 12.0 days half-life -- isomeric transition to a stable isotope; PA1 133 -- 5.27 days half-life -- isomeric transition to a stable isotope; PA1 133m -- 2.3 days half-life -- isomeric transition to Xe 133. PA1 135m -- 15.3 minutes half life -- isomeric transition to Xe 135 of half-life 9.2 hours. PA1 135 -- isomeric transition to isotope of half-life 2.6 million years. PA1 85 -- 10.6 Yrs. half-life -- isomeric transition to a stable isotope; PA1 85m -- 4.4 hours half-life -- isomeric transition to Kr85 or to stable isotope; PA1 87 -- 78 minutes half-life -- isomeric transition to an isotope of half life 5 .times. 10.sup.10 Yr; PA1 88 -- 2.77 hours half-life -- isomeric transition to an isotope of half life 17.8 min.
The Kr isotopes have half-lives as follows:
Typically, the primary fluid of a PWR reactor has a volume of about 6000 to 12,000 cubic feet and while in operation may contain up to about 2.0 standard cubic ft. of the Xe fission product isotopes and 0.5 standard cubic feet of Kr fission product isotopes. This volume of fission product isotopes includes a large proportion of stable isotopes and a small proportion of radioactive isotopes. The predominant radioactive isotope present is Xe.sup.133.
Before refueling or like operations the nuclear reactor is shut down but before the reactor becomes accessible for the actual refueling or other work the concentration of these readioactive isotopes must be reduced to a permissible level to avoid discharges of radioactive effluents into the environment of the reactor and increased doses of radioactivity to refueling personnel. While the half-life of these isotopes is relatively short, it is measured in days and hours and access to the reactor for refueling is delayed by their presence for a day or several days.
In accordance with the teachings of the prior art the primary fluid is circulated through a flash tank during normal operation of a reactor and the stripped fission gases are purged periodically or continuously. Before refueling the reactor is shut down but the circulation and purging continues. At very low fission gas concentrations required for refueling, this process is slow and inefficient. There is no provision to enhance the rate of dissolution in the flash tank other than by increasing process flow rates; this is relatively ineffective and costly.
It is an object of this invention to overcome the above-described disadvantages of the prior art and to increase the availability of a nuclear reactor for power generation by reducing substantially the time after shut down of a nuclear reactor which is consumed in waiting for the radioactivity arising from the gaseous fission products to drop to the required or established low level.
It is a further object of this invention to overcome the disadvantages of the prior art effectively and at low cost without adding additional equipment to the present nuclear-reactor plant.